For the separations of ternary/quaternary
reacting mixtures with
the most unfavorable relative volatilities (i.e., the reactants are
the lightest and the heaviest components with the product/products
as the intermediate one/ones), the reactive distillation column with
a top-bottom external recycle (RDC-TBER) was already demonstrated
to be considerably advantageous over the currently available process
schemes [Chen, H.; Huang, K.; Liu, W.; Zhang, L.; Wang, S.; Wang,
S. J. Enhancing Mass and Energy Integration by External Recycle in
Reactive Distillation Columns. AIChE J.
2013, 59 (6), 2015]. In the present study, the RDC-TBER
is further evaluated in terms of the separations of two more complicated
reacting systems: the adipic acid and glutaric acid esterifications,
which feature a two-stage cascade reaction kinetics besides the most
unfavorable ranking of relative volatilities (i.e., the relative volatilities
of the intermediate and final products are within those of the lightest
and heaviest reactants). In comparison with the flowsheet by conventional
reactive distillation column, the flowsheet by the RDC-TBER leads
to sharply reduced capital investment, operating cost, and total annual
cost (TAC). Moreover, the reduction of the TAC tends to increase in
case the two-stage cascade reaction involved becomes difficult to
proceed. These outcomes indicate that the arrangement of a top–bottom
external recycle helps to enhance internal mass integration and/or
internal energy integration between the reaction operation and the
separation operation involved and facilitates the RDC-TBER to be a
favorable option for the separations of adipic acid and glutaric acid
esterifications.